When properly applied in a hydraulic circuit, bladder and diaphragm accumulators can have a long and trouble-free life. But if their operating parameters are not correct, recurring failure can result. Consider this story from one of our members:
“Currently we are using a miniature accumulator from Hawe — part number AC 130-1/4, max. operating pressure 500 bar, max. gas fill (nitrogen) 250 bar, with a rated volume of 13 cm3. We charge these accumulators to 6 bar and use them on a circuit with 9 bar supply pressure. These circuits are used to test the control pressure on variable solenoids in a transmission control module. We are using Dextron 6 ATF for our hydraulic fluid. We have 8 testers that have 6 of these accumulators on them – 48 accumulators total with more testers to come in the near future. My problem is that we have to replace about 3 accumulators a week because they have ruptured. Is there something I can do to fix this problem?”
There are a couple of things which should be checked when a bladder or diaphragm accumulator fails. The first is compression ratio. If the bladder or diaphragm is subject to excessive deformation when the accumulator is pressurized to maximum system pressure, the life expectancy of the bladder or diaphragm is greatly reduced.
Compression ratio is calculated by dividing the maximum pressure of the system or sub-system in which the accumulator is installed (P2), by its gas pre-charge pressure (P0).
In the example above, P2 = 9 bar and P0 = 6 bar, so the compression ratio for the application is P2/ P0 = 9/6 = 1.5 to 1. The permissible compression ratio for a bladder accumulator is typically 4 to 1 and 6 to 1 for diaphragm units, so this is well within acceptable limits. A quick look at the data sheet for a Hawe AC 130-1/4 accumulator confirms this. The AC 130-1/4 is a diaphragm accumulator with an allowable compression ratio of 4 to 1.
The next thing to check is that the actual gas pre-charge pressure is correct. Depending on the application of the accumulator, pre-charge pressure (P0) is typically 0.6 to 0.9 of the minimum pressure of the system or sub-system in which the accumulator is installed (P1).
From a reliability perspective, the reason why P0 must always be less than P1 is so that the accumulator is never completely emptied of fluid during normal operation. If all fluid is discharge from the accumulator at minimum system pressure (P1) there is a risk of the bladder or diaphragm being damaged by the anti-extrusion device. This is a device which is designed to stop the bladder or diaphragm entering the accumulator’s discharge port.
In practice this means that if minimum system pressure (P1) is changed for any reason, pre-charge pressure (P0) must also be changed. And if P0 is changed, the compression ratio, P2/ P0 should be re-checked.
In the example above, we’re told that P0 = 6 bar and P2 = 9 bar, but we’re not told what P1 is. However, by reverse calculation we can say that if minimum system pressure falls below 6.7 bar, then the diaphragm is at risk of being damaged due to complete discharge of fluid from the accumulator.
Another consideration in the above application is operating temperature. Accumulator bladders/diaphragms are made from polymers, and the life of all polymeric materials is reduced exponentially by operating temperatures which exceed 80°C.
We’re not told what the fluid operating temperature is during testing, but given this is an automatic transmission test rig, and automatic transmissions can operate with peak temperatures as high as 150°C (300°F), this is much higher than what is typical or ideal for a conventional hydraulic system. So excessive operating oil temperature may also be a factor in diaphragm failure in this case.
And while on the subject of temperature extremes, remember when charging the gas end of a bladder or diaphragm accumulator, the nitrogen gas should always be admitted very SLOWLY. If the high pressure nitrogen is allowed to expand rapidly as it enters the bladder, it can chill the bladder’s polymer material to the point where immediate brittle failure occurs. Rapid pre-charging can also result in the bladder or diaphragm being damaged by the anti-extrusion device. And to discover six other costly mistakes you want to be sure to avoid with your hydraulic equipment, get “Six Costly Mistakes Most Hydraulics Users Make… And How You Can Avoid Them!” available for FREE download here.